In the manufacture of patterned devices such as semiconductor chips and carriers, the steps of defining different layers which constitute the desired product are among the most critical and crucial steps involved. Polymer films are often used in integrated circuit fabrication as a pattern transfer mask for the semiconductor substrates. For example, polymers used as a photoresist can act as a mask for etching, ion implantation, or lift-off to induce designated removal, doping, or addition to the underlying substrate, respectively.
As the lines and spaces to be etched, however, become smaller, such as at about 1 micron, the photolithographic procedures for producing the photoresist pattern that is the etch mask are affected by such parameters as reflections from the surface grain structure of the metals or polysilicon substrate to be etched, standing wave effects, variations in the photosensitive material thickness, reflections from steps and diffraction effects.
One technique for overcoming the problems of surface topology, reflections and diffractions, is to employ a multi-layer resist system known as a portable conformable mask (PCM) system. Such is described by Burn Jeng Lin, "Portable Conformable Mask --A Hybrid Near U.V. and Deep U.V. Patterning Technique", Proceeding of SPIE, Vol. 174, page 114, 1979, disclosure of which is incorporated herein by reference. The simplest multi-layer resist system employs a two-layer resist system which avoids the cost and complexity of most other multi-layer systems. The bottom layer is insensitive to the radiation used to image the top resist layer, and is preferably a resist from a polymer of methylmethacrylate such as polymethylmethacrylate (PMMA), that is applied over the wafer topology to provide a planar surface. The top layer is generally a relatively thin (e.g., about 1 micron or less) layer of a material that is simultaneously sensitive to the imaging radiation --electron beam, X-ray, or optical radiation --and opaque to the radiation used to expose the bottom layer. Typically this can be a positive photosensitive material that responds to the imaging radiation such as ultraviolet light used in step-and-repeat photolithography and is opaque to deep U.V. wavelengths used to expose PMMA. After the top layer is imaged and developed, the bottom layer is imaged by flood exposure through the top layer resist mask and developed using, for instance deep U.V. (about 190 nm to about 280 nm).
A number of the suggested processes used to overcome image distortion or linewidth variation due to different radiations from the irregular surfaces require processing that is rather complex. This causes a decrease in the throughput and an increase in process control problems.
It has also been suggested to use two layers of the same parent resist of varying molecular weights to achieve different sensitivities or solubilities. However, such an approach requires unique synthetic effort for control of molecular weight as well as tedious polymer characterizations. It would, therefore, be desirable to provide a method that is capable of overcoming image distortion or linewidth variations due to different radiations from irregular surfaces that does not require the complex processing now employed.